Marshall MG100 HDFX Burnt IC1

[DonJoe]

Dear friends:

I recently bought a Mashall MG100 HDFX head on auction. The seller's description tells that the IC1 chip is burnt. I allready downloaded the schematic from ths forum. It seems to be an easy repair. I allready bought the 4558DD from a parts distributor. My concern is what would cause the chip to burn. Should I check the voltage regulators REG1 & REG2, the capacitors or just replace the chip and power on. My 13 old and I really enjoy learning the electronics and playing toghether, but we would be very sad if this turns out to be a dud. We'll really apreciate your help on this. Just point us in the right direction, will do the rest.

[J M Fahey]

What happens if you power the amp as-is?

[52 Bill]

I allready bought the 4558DD from a parts distributor. My concern is what would cause the chip to burn.

My guess would be that this is not the dead chip in your new amp.

Much more likely the output power amp IC.

[Enzo]

Rather than deal in part numbers, why not tell us the problem the amp has.


Parts fail, and if some IC in the middle of the circuit dies, it is not likely due to power supply failures. besides, you can measure the power rails to the ICs. If they measure 15v or whatever, then the Vregs are doing their job.

As Bill points out, the failed chip is much more lukely to be the power amp, that TDA7293V. AFter all, it is IC1 on the little power amp board. Much more likely than IC1 in the middle of the preamp. And when TDA7293 fails, it often "burns," which is not usually a fitting description of a preamp IC failure.

Mouser has 773 TDA7293V in stock at $4.06 each. I bet they would send one of them to you.

[DonJoe]

Thanks for your prompt responses

Actually we have not received the amp from the seller. We are still waiting for it to be delivered. As soon as we get it, we will go over all your suggestions and report back. Again you all are very kind. I am glad I found this forum.